Project Summary The major cause of death in systemic lupus erythematosus (SLE) is lupus nephritis (LN) but the mechanisms responsible for kidney damage remain only partly understood. A limitation of current mouse models of LN is that genetic causes are incompletely understood or they are single gene models that don?t reflect the polygenic nature of human SLE. To generate a polygenic mouse model with immune pathway abnormalities that are known to underlie SLE, we generated mice with defects in apoptotic cell (AC) clearance (lack MFG-E8), have low complement (C1q or C3 KO) and that produce anti-chromatin antibodies (have Sle1 interval). We observed that, only when all three components are present together (triple mutants, TM) do mice get LN. Of great interest, the C3 deficient TM was sensitive to UV light and developed chronic skin disease and systemic manifestations following UV exposure. The long-term goals of this project are: to understand the protective role of the AC opsonin, MFG-E8, in the kidney; to explore the protective roles of complement in the kidney under conditions of nephritogenic antibodies and impaired AC clearance; to identify the effector mechanisms of kidney injury in TM mice and the mechanisms responsible for UV mediated kidney disease. In Aim 1, we will determine whether AC and inflammation are increased in the kidneys of double versus single KO mice. We will also test whether MFG-E8 has anti-inflammatory and anti-fibrotic effects in the kidney by reconstituting mice with MFG-E8 or MFG-E8 conditioned macrophages. In Aim 2, we will determine the effector mechanisms responsible for kidney injury. Based on preliminary data, we will confirm that in low C1q states the lectin pathway is activated by DAMPS and will attempt to prove lectin pathway pathogenicity by blocking Masp- 2. In low C3 states where we observed C5-9 formation, we will test whether an alternate C5 convertase is generated by thrombin and will attempt to prove pathogenicity by inhibition experiments. These pathways will be validated using uniform nephrotoxic sera in mice deficient in Mfge8, C1q, C3 or double knock outs (DKO) and pilot studies will be performed in kidneys from SLE patients with persistently low complement. Aim 3 takes advantage of the observation that UV light exposure of skin in C3 TM mice leads to systemic disease. We will examine whether this susceptibility is explained by release of oxidized mitochondrial DNA from neutrophils and/ or by trafficking of inflammatory cells from skin to kidney, including by the process of reverse transmigration. Overall, using novel strains that accurately reflect pathway abnormalities in human SLE, we will determine whether and how early complement components as well as an opsonin such as MFG-E8, protect the kidneys. The involvement of the lectin pathway or thrombin have important therapeutic implications as therapies to block each of these pathways either are, or can be used, in humans. Identification of the mechanisms responsible for UV triggered kidney injury will also lead to therapeutic approaches such as inhibition of mitochondrial ROS or inhibitors of cGAS-STING or inhibitors of neutrophil reverse transmigration.